A pushbutton switch of this type is disclosed in Japanese Utility Model Publication 58-164138A, and it is shown in a cross-sectional view of FIG. 6 of this application.
Referring to FIG. 6, reference numeral 1 designates a switch housing which is a synthetic resin mold in the form of a box opening at its upper end. Along an inner bottom surface la of the housing 1 are integrally provided stationary contacts 2a--2a and 2b in such a manner that their upper portions are exposed above the inner bottom surface 1a. Terminal extensions 3 extend from the stationary contacts 2a--2a and 2b to the exterior of the switch housing 1. Reference numeral 4 denotes a movable contact in the form of a dome-shaped leaf spring. The movable contact 4 is held in the switch housing 1 and overlies the inner bottom surface 1a. Parts of free ends 4a which form a peripheral margin of the movable contact 4 are located on the stationary contacts 2a--2a, and the movable contact 4 and the stationary contacts 2a--2a are held in continuous contact. Reference numeral 5 denotes a stem which is held in the switch housing 1 movably in the up-and-down direction. A projection 5a provided at the center of the lower surface of the stem 5 is held in contact with the top of the movable contact 4. Reference numeral 6 designates a cover which is clamped and fixed so as to overlie the upper opening of the switch housing 1 and has an aperture 6a which permits an upper projection of the stem 5 to project to the exterior of the switch housing 1 through the aperture 6a.
Under this arrangement, the prior art pushbutton switch operates as explained below.
As shown in FIG. 6, the stem 5 (shown by a solid line) is held in a close contact with the lower surface of the cover 6 by the movable contact 4 (shown by a solid line), and the central portion of the movable contact 4 remains apart from the stationary contact 2b located thereunder. Therefore, the stationary contacts 2a--2a are not in electrical conduction with the stationary contact 2b, namely, the switch exhibits its switch-off condition.
When the stem 5 is pushed down from the position of FIG. 6, the projection 5a of the stem 5 urges the top of the movable contact 4. Responsively, the movable contact 4 gradually yields downwardly, and it finally exhibits an inverted configuration. Then the movable contact 4 takes the configuration shown by a two-dot-and-dash line in FIG. 6 in which the top of the movable contact 4 contacts the stationary contact 2b thereunder and the stationary contacts 2a--2a and stationary contact 2b are electrically connected via the movable contact 4. Thus the switch exhibits its switch-on condition.
When the pushing force is removed from the stem 5, the movable contact 4, heretofore held in the inverted configuration shown by the two-dot-and-dash line in FIG. 6, demonstrates its revival force and restores its original configuration in which the top thereof is located at an upper position shown by the solid line. Thus the switch is returned to its switch-off condition.
The aforementioned prior art is configured to move the movable contact 4 into or out of contact with the stationary contact 2b thereunder by effecting inverting motions of the movable contact 4. However, in order to meet a recent demand of reduction in weight, thickness and length of pushbutton switches, the stationary contacts 2a--2a must not project upward so much from the inner bottom surface 1a of the switch housing 1. Therefore, when the top of the movable contact 4 invertingly moves into or out of contact with the central stationary contact 2b in response to up and down movements of the stem 5, free end portions of the movable contact 4 opposed to the inner bottom surface 1a often hit the inner bottom surface 1a and scrape off the inner bottom surface 1a. Scraped resin powder often causes unreliable electrical conduction between the contacts.